The original question had to be edited. Overall, the sniper's appearance is consistent with a solitary soldier keeping vigil over his intended target. He is a young solider, brimming with the excitement that young soldiers would possess over war and "seeing action:"
Beside him lay his rifle and over his shoulders was slung a pair of field glasses. His face was the face of a student, thin and ascetic, but his eyes had the cold gleam of the fanatic. They were deep and thoughtful, the eyes of a man who is used to looking at death.
He was eating a sandwich hungrily. He had eaten nothing since morning. He had been too excited to eat. He finished the sandwich, and, taking a flask of whisky from his pocket, he took a short drought.
Such descriptions help to capture the youth and relative purity of mission within the sniper. He was "thin and ascetic," helping to enhance the idea that this mission was the only purpose to his being. At the same time, the "cold gleam of a fanatic" represents how fully immersed the sniper is in his mission, singular in his focus with nothing to possibly deter him from what it is he knows he must do. In this, his appearance is completely aligned with what a young, ruddy soldier would look like in his condition.